Both in the case of the "genuine radar method" and in the case of the Time Domain Reflectometry (TDR) method, measuring signals are transmitted in the direction of the surface of a medium and at least partially reflected at the surface of the medium as so-called echo signals. The reflected echo signals are detected and evaluated by means of a delay-time method. The main difference between the genuine radar method and the TDR method is that in the first case the electromagnetic waves, or the sound or ultrasound waves are freely emitted, while in the case of the TDR method the electromagnetic or acoustic measuring signals propagate in a directed manner along an element guiding the measuring signals. By evaluation of the amplitude values and possibly the phase values of the echo signals, the filling level in the container is determined by means of a delay-time method. Both the pulsed radar method, in which the echo signals are emitted in a pulsed manner, and the FMCW method, in which continuous waves arc frequency-modulated in a periodically linear manner, for example with a sawtooth voltage, may be used. In the case of the TDR method, usually steep-edged delta pulses are sent to the conductive element.
Delay-time methods use the physical law according to which the transit distance is equal to the product of the delay time and the propagation velocity. In the case of filling level measurement, the transit distance corresponds to twice the distance between the antenna and the surface of the filled product. The actual useful echo signal and its delay time arc usually determined on the basis of the so-called echo function or the digital envelope curve, the envelope curve reproducing the amplitudes of the echo signals as a function of the "antenna - surface of the filled product" distance. The filling level itself is then obtained from the difference between the known distance of the antenna from the bottom of the container and the measured distance of the surface of the filled product from the antenna. Analogous considerations apply to the determination of the mass how through a line.
Instruments of the type referred to above are manufactured and sold by Endress+Hauser in the area of filling level measurement and flow measurement. For instance, the product designation "Mikropilot" stands for a sensor which freely emits microwaves; "Prosonic" or "Prosonic Flow" identify sensors which operate on the basis of ultrasound waves, and "Levelflex" designates a TDR sensor.